Thermal Radiation

Thermal radiation is electromagnetic radiation emitted from the surface of an object, which is due to the object's temperature, and is generated by the movement of particles within molecules. Any object that has a temperature above 0 K (Kelvin), such as Earth and the sun (and me and you), emits this kind of radiation (called "thermal" because it comes from "warm" bodies).  Matter not only emits electromagnetic radiation, it also absorbs some of the electromagnetic energy that impinges on it and, as a result, warms up (i.e., it's temperature increases). In this way, "heat" is transferred through electromagnetic radiation, as in the way that Earth warms as a result of the EM energy it recieves from the sun.  (Heat can also be transferred by convection and conduction, but we aren't going to talk about those processes here). If an object is in radiative equilibrium with its surroundings, the amount of EM radiation absorbed will equal the amount emitted and the object's temperature will stay constant.